Cathode for electric discharge tubes



Sept. 1, 1959 A. LlEB CATHODE FOR ELECTRIC DISCHARGE TUBES Filed Nov. 25, 1955 Fig. 4

Fig. 5

INVENTOR A. Ll E8 ATTORNEY United States Patent CATHODE FOR ELECTRIC DISCHARGE TUBES Albert Lieb, Oberesslingen (Neckar), Germany, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Application November 23, 1955, Serial No. 548,684 Claims priority, application Germany November 23, 1954 3 Claims. (Cl. 313-470) The invention relates to cathodes in electric discharge tubes, in particular in cathode-ray tubes, and in which the entire cathode sleeve, during the operation, will move in such a way that at all times emissive parts of the cathode are utilized for the discharge process. From the movements of the cathode by heat expansion there results the advantage that in those cases in which the emissivity is reduced by drawing the discharge current, along different portions of the emissive surface of the cathode during the operation will be either improved, or the service life of the discharge tube will be increased.

Movable cathodes in cathode-ray tubes to maintain correct position are already known according to the prior art. One such proposal suggests to move the cathode by the tension of a spring. This spring consists of a material which is decomposed under the influence of the current load, and the thickness of the wire is decreased and which, consequently, loses its tension force or elasticity. According to another proposal there is provided a strip of bi-metal which, under the influence of the heat, moves the cathode.

Further embodiments have been suggested in which the movement is effected from the outside of the tube with the aid of a magnet. All of the conventional arrangements have the disadvantage of requiring for this purpose special, additional means not required for the discharge process itself, such as springs, toothed wheels, bi-metal strips, magnets and the like.

According to an object of this invention the movement of the cathode is effected without the employment of such additional means. Therefore, in accordance with the invention, the cathode may be fixed at one supporting point in such a way that, on account of the heat expansion when its temperature increases it, a certain movement results in a direction toward another point nearer the filament will be heated quicker than the other supporting points during the heating process, or that the heat expansion of the cathode at one of the supporting points is greater than the heat expansion of the supporting body, or that the cathode is more tightly attached to the retained supporting point closer to the filament than to the other supporting points, or that the supporting surface of the cathode at the retained sup porting point is greater than at the other supporting points.

With respect to the known arrangements this invention bears the added advantage that a movement of the cathode is effected without additional mechanical or extraneous means and occurs automatically, upon switching on the heating current or when modifying the heating of the cathode. In regard to some special cases of practical application it may be of some advantage to not move the cathode during the normal switching-on process. In this case the movement, as another feature of the invention occurs only when there is a momentary over-heating resulting from the normal heating process. Since no additional mechanical or structural arrangements ice for moving the cathode are required, unlike the previous conventional arrangements, the cathode of the invention has the advantage of being particularly simple and economical.

The invention is particularly suitable for the employment in tubes, e.g. cathode-ray tubes, in which a damaging of the cathode may be effected by the gas ions. These disturbing gas ions appear principally during the early period of operation of the tubes. According to the invention, the cathode arrangements is such that the emitting layer which delivers the emission current during normal operation is prevented from being previously damaged by gas ions. The position of the emitting layer located on a cathode sleeve which surrounds the cathode filament will move to various positions relative to the opening in the Wehnelt cylinder 4 during the heating period. As a rule, however, it will be sufiicient to permanently retain the cathode in its finally reached position after having achieved a sufiiciently good vacuum. For such cases the invention provides a mechanical limiting stop to terminate the movement of the cathode.

One embodiment of the invention for use with a cathode arrangement of a cathode-ray tube is shown in Figs. 1 and 2 of the accompanying drawings.

Fig. l of the drawing shows a longitudinal section and Fig. 2 a cross-sectional view of the arrangement.

A further embodiment of the invention is: schematically shown in Figs. 3, 4 and 5 of the drawing.

An elongated sleeve cathode 1 is supported by the disks 5 and 6. These disks may consist of mica. The emitting layer of the cathode is denoted by the reference numeral 2. The disks 5 and 6 are provided with openings fitted to the profile of the sleeve cathode 1 in the non-heated conditions so that the cathode may be slid into position with respect to the disks. The emitting layer 2 and the Wehnelt cylinder 4, are so positioned in the tube assembly that only that portion of the emitting layer of the cathode which is denoted by 3, will be in front of the opening of the Wehnelt cylinder 4. The filament 7 is positioned further from the supporting point of the disk 5 than the supporting point of the disk 6. Thus the supporting point at the disk 6 will be heated more quickly upon initiating the heating process than the supporting point at disk 5. Because of the radial expansion of the cathode, which is heated quicker than the disk 6, the cathode, at the supporting point of cathode sleeve 1 and disk 6 will be retained by the radial expansion. Accordingly, during the subsequent further heating of the cathode, the further longitudinal expansion will be efiiected only in direction toward the disk 5. In another embodiment such as shown in Figure 3 the play between the supporting disks 5' and 6 and the cathode is so dimensioned that movement in one direction will occur only during the heating process and not during the slow-cooling-down process. In the latter case the retaining of the cathode cannot be effected be cause the necessary strong radial heat expansion differences of the disks 5 and 6 and the cathode appear at none of the supporting points. In other words upon the slow cooling down process both ends of cathode sleeve 1 will be released from the grip of their respective support disk at more nearly the same time. Thus the portion 3 of the emissive layer will not return to the same position as it had before heating beneath the opening in the Wehnelt cylinder 4. Therefore, a creeping action of the sleeve occurs in a direction toward disk 5 when the filament is turned on and 01f.

The retaining of the cathode at one supporting point is eiiected or, at least, can be improved by arranging it so that the heat expansion of the cathode, at the retained supporting point, is greater than that of the supporting body, or that the fitting of the cathode at the retained supporting point is tighter than at the other supporting points or that the supporting surface of the cathode at the retained supporting point is larger than at the other supporting points.

To prevent the cathode from moving beyond the disk 6 it is further provided to limit the movement by means of a mechanical limiting stop 8 adjacent one end 9 of sleeve 1. In the example illustrated this limiting stop is designed as a so-called roll or bead, that is pressed out of the cathode. Another limiting stop which may be used consists in attaching the one end of the cathode supply leads to the cathode in such a way that it will serve to terminate the movement of the cathode and, thus acts as a limiting stop. This can be ascertained from Figure 1, the length of the emitting layer 2 is equal to the length between support 6 and limiting stop 8. Thus the creeping of the cathode sleeve 1 will never result in the sleeve moving past the limiting stop 8.

In the other embodiment Fig. 3 shows a longitudinal section, and the Figs. 4 and 5 a cross-sectional view through the cathode system of a cathode ray tube. The reference numerals for the parts of the cathode system are the same as those employed with the embodiment shown in Figs. 1 and 2 of the drawing.

In the example of embodiment as shown in Figs. 3 to 5 the cathode sleeve 1 is supported by disk 5 only at some selected points or portions of the surface, but is supported by disk 6 along the entire cathode periphery. In the case of a rapid heating of the cathode, the cathode sleeve 1 is expanded, at least momentarily, against the disks 5 and 6. The fitting of the cathode with the disk 6, that is the force, which retains the cathode in the disk, becauseof the larger surface area and the smaller elasticity of the support in each phase or stage of the heating process, is substantially greater than with the disk 5. In this way longitudinal expansion of the cathode produces a sliding movement of the cathode in direction toward the disk 5. From this sliding movement there results the displacement of the emitting layer 2 with respect to the opening in the Wehnelt cylinder 4.

What is claimed is:

1. An indirectly heated cathode system for a cathode ray tube comprising an elongated sleeve, first and second supports at opposite ends of said sleeve, means for supporting said sleeve in openings in said first and second supports substantially conforming to the peripheral contour of said sleeve, a heating filament positioned within said sleeve, an emissive coating on one surface of said sleeve, an element having an aperture smaller in area than said emissive coating, said element being so positioned in spacial relation from said coated surface so as to block emission from said emissive coating except through its aperture, and means for causing said first support to grip said sleeve before said second support when said sleeve expands radially upon the energization of said filament. g Q p 2. The cathode of claim 1, wherein said heating" filal ment is positioned closer to said firstsup'port than to said second support so that initial heating of said sleeve adjacent said first support is greater. 7 I

3. A' cathode of claim 1 wherein the' m'eai'i's for cans:

ing one support to grip before the other support upon heating of the sleeve comprises said apertures in said first support having more frictional engagement with said cathode sleeve than the other aperture in said second support.

References Cited in the file of, this patent UNITED STATES PATENTS 1,866,715 Krahl July 12, 193 2 1,980,572 Callahan NOV. 13, 1934 2,116,788 Haslauer May 10, 1938 2,164,547 Smith July 4, 1939 2,206,511 McLaughlin et al. July 2, 1940 2,527,166 Walsh Oct. 24, 1950 

